1. Technical Field
This invention relates to the field of computer tomography and radiography, and more particularly to a detection apparatus and method involving a plurality of flat radiation detectors having an associated semiconductor blocking junction.
2. Description of the Prior Art
Arrangements for making a tomogram are known where a fan-shaped beam of ionizing rays passes through the image elements in the body section plane of a subject to be radiated successively in different directions. Correlated with the beam is a detector arrangement with a plurality of detectors arranged in the body section plane side by side. The conversion of the various single roentgenograms of the body elements and their correlation with the respective image element of the tomogram to be produced are obtained by means of an electronic system which contains a computer. These known arrangements, in which the radiation source with the detector system in the body section plane is rotated about the body step by step, are known in the art as computer tomograms and are disclosed in general and particular detail in U.S. Pat. No. 3,974,388.
Further arrangements for making roentgen shadowgrams of an object, preferably of a human body, are disclosed in U.S. Pat. No. 4,174,481 where, too, a detector system is correlated with a fan-shaped beam of ionizing rays, but where the body is moved relative to the beam after each radiography step by step normal to the fan of the beam. The detectors, therefore, arranged in the radiation plane, successively receive the absorption values of parallel layers of the body. Such arrangements for making roentgen shadowgrams with a movement of the body perpendicular to the fan of the beam are known in the art under the general desgination of computer radiographs.
In computer tomography (CT) and in computer radiography (CR), a detector system with several hundred and often more than one thousand, detectors detects the ionizing radiation, in particular x- or gamma-radiation attenuated by the human body. The particular detector system must document, that is, absorb, the radiation differently attenuated by the individual body parts as completely as possible. Further, the electronic noise of the detector with a particular electronic amplifier type should be small in relation to the quantum noise .sqroot.N of the minimum roentgen quantum number N occurring during the measuring time, especially at maximum attenuation when the signal to noise ratio is most important. Besides, the detectors should be as fast as possible in a rapid computer tomography apparatus with continuous x-radiation, and, moreover, there should be no afterglow or persistent current which might falsify the x-ray picture.
It is known to use a scintillator-photodiode combination with silicon photodiodes in computer tomography and computer radiography detector systems. Also xenon chambers are sometimes used as detector systems for x-rays. For a high-resolution computer tomography apparatus with continuous x-radiation, however, no known detector fulfills the objectives of the present invention. Cesium iodide, CsI, is generally used in a scintillator-photodiode combination, and while it radiates high signals, it exhibits a disturbing afterglow. Other known scintillators radiate signals smaller by a factor of 4 to 5. Xenon permits only an incomplete absorption of the x-rays. Other problems with xenon are high pressures, high fields at small electrode distance, and problems with a tottering focus of the x-ray tube.
The present detector system for computer tomography contains semiconductor detectors with a flat semiconductor body of n-conducting silicon, whose flat sides are each provided with an electrode, one of which forms a blocking junction designed as a metal-semiconductor junction. One of the end faces of the semiconductor body, about 1.5 mm wide and about 17 mm long, is provided for receiving the radiation. The depth of the semiconductor bodies in the radiation direction is selected to be relatively deep and may be about 20 mm as taught in the publication, "Multichannel Semiconductor Detectors for X-Ray Transmission Computed Tomography," appearing in IEEE Transactions on Nuclear Science, Vol. NS-27, No. 1, February, 1980 at pages 252-257. Silicon, however, absorbs only a relatively small portion of the ionizing rays. Even by increasing the depth, for example, to about 40 mm in the direction of the arriving rays, the absorption is not sufficiently complete.
It is further known that detector systems with semiconductor detectors of socalled highest purity or very pure germanium with a very small dopant concentration of, for example, only about 10.sup.11 to 10.sup.9 atoms/cc are suitable for computer tomography. In a known embodiment of such a detector system, however, operation of the system must be at a cryogenic temperature around 80 K. and in a vacuum because of the narrow band gap of the germanium detectors.
It is also known that nuclear radiation detectors of highest purity germanium of large volume can, in one particular embodiment, indeed be stored at a higher temperature as disclosed in German Pat. No. 25 46 451. However, it has been documented by precise measurements and illustrated by diagrams that germanium detectors can be operated only at temperatures below 150 K. as disclosed in the publication, "Operational Characteristics of Germanium Detectors at Higher Temperatures," appearing in IEEE Transactions on Nuclear Science, Vol. NS-20, No. 1, February, 1973 at pages 494-499 (9 D 02).